1. Field of the Invention
The present invention relates, in general, to the adhesive bonding of two surfaces together, and, more particularly, to a process for bonding a solar cell cover glass to a solar cell, and for bonding solar cells to an underlying substrate.
2. Description of Related Art
Materials which are used in a space environment must be able to withstand extreme conditions, such as exposure to temperatures from about -70.degree. to +120.degree. C. for extended periods of time, such as ten years, and exposure to vacuum conditions in space. In addition, materials in space are exposed to high energy ultraviolet radiation, high energy protons, and high energy electrons, all of which can produce significant damage.
In the case of solar cells used in space, a cover glass, usually formed of silica, is bonded to the front of each cell to protect the photovoltaic junction from radiation and particle damage. The cover glass also acts as a thermal control link to allow the solar cells to operate at lower temperatures. However, even with such a glass cover, high energy electron damage does occur. It has been found that the effect of electron radiation can be minimized if the cells are run at higher temperatures or periodically cycled to higher temperatures. In the case of silicon solar cells, electron damage can be annealed and the cell efficiency optimized by heating the cells to about 60.degree. to 100.degree. C. At these temperatures, known adhesives used to bond the cover glass to the solar cell, such as dimethyl silicone resins, are sufficient.
However, solar cells based on gallium arsenide semiconductor technology are currently being developed which have a much higher efficiency than silicon solar cells at increased temperatures. In order to anneal the gallium arsenide cells for electron damage, a temperature in the range of about 250.degree. to 350.degree. C. is required.
Commercially available adhesive systems which are currently used in space hardware environments include DC-93-500, a liquid two-part room temperature vulcanizing dimethyl silicone resin available from Dow Corning, which is used for bonding of cover glasses to solar cells. This two-part liquid is mixed, and then within one hour, the liquid must be placed between the substrates and the bond formed between the cover glass and the solar cell.
In the bonding of solar cells and other parts to the underlying spacecraft, a filled liquid rubber, such as RTV 566 or RTV 567, available from General Electric Company, is used.
While these commercially available materials allow thermal excursion of the bondlines to between 200.degree. C. and 300.degree. C., a need remains for an adhesive that can withstand annealing temperatures up to at least 350.degree. C. and which has the advantage of low temperature vulcanizing.
The cost, inconvenience, and hazardous waste generated by the bonding of cover glasses to solar cells and the bonding of solar cells to supporting substrates with a liquid rubber having a short shelf life are considerable. Accordingly, a new bonding process is desired.
One material that has been previously studied for its thermal stability is a class of ultra-high molecular weight carborane siloxane polymers, as described by D. D. Stewart et al in "D2-m-Carborane Siloxanes. 7. Synthesis and Properties of Ultra-High Molecular Weight Polymer", Macromolecules, Vol. 12, No. 3, pp. 373-377 (May-June 1979). In the method of Stewart et al, the carborane siloxane polymers are formed by:
(1) forming a slurry of carborane bisdimethyl silanol in dried chlorobenzene and cooling the slurry to -10.degree..+-.5.degree. C.;
(2) adding to the slurry a mixture of dimethylbisureido silane and methylphenylbisureido silane to form a reaction mixture at -10.degree..+-.5.degree. C.;
(3) separating from the reaction mixture a silanol end-capped prepolymer of the polycarborane siloxane polymer;
(4) dissolving the prepolymer in chlorobenzene to form a solution; and
(5) adding to the prepolymer solution an excess of the above-noted bisureido silanes.
Stewart et al reported that the polymers so formed had molecular weights in excess of 10.sup.6, which was believed to be due to the technique of the reverse addition of the bisureido silanes to the carborane disilanol in chlorobenzene. However, as disclosed by Stewart et al at page 375, right column, first and second full paragraphs, consistent and reliable results were not achieved. One problem was that a reliable technique for purifying the polymer was not found, and consequently the prepolymer was degraded by reaction with amine by-products. Another problem was that many of the prepolymer samples were not capable of being advanced to the high molecular weight polymers, and no cause for this difficulty was defined. In addition, attempts to replicate the experiments of Stewart et al did not result in polymers of the highest molecular weight reported by Stewart et al. As discussed by Stewart et al, optimum mechanical and thermal properties of these polymers occur only at high molecular weights.
Since the carborane siloxane polymers of Stewart et al at high molecular weight have desirable high temperature properties and could be useful in a space environment, the present inventor developed methods of reproducibly forming a very high molecular weight polycarborane siloxane polymer. These methods are disclosed and claimed in copending application Ser. No. 07/807,364, filed Dec. 16, 1991, now U.S. Pat. No. 5,208,310 and Ser. No. 07/895,061, filed Jun. 8, 1992, and assigned to the same assignee as the present application.
While the new methods result in a considerably improved polymer over Stewart et al, the base polymer lacks the required mechanical integrity at high temperature to allow its use in adhesive bonds for space-stable structures. Thus, it is desired to provide improved high temperature stability compared to the base polymer. It is especially desired to use the improved polymer to adhere one substrate to another, such as solar cell cover glasses to solar cells or solar cells to underlying substrates.